This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-012031, filed Jan. 19, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a hydrodynamic bearing for a motor, which is intended to extend bearing life in a hydrodynamic bearing having a groove for generating a dynamic pressure provided thereto.
2. Description of the Related Art
In recent years, increase in density and speed and reduction in price have been demanded in disk drive apparatuses or polygon mirror drive apparatuses, and an increase in accuracy, prolongation of life under high-speed operation and reduction in cost are required in bearings for the drive motors of these apparatuses.
Prolongation of bearing life has been conventionally attempted, and Jpn. Pat. Appln. KOKAI Publication No. 7-109215 has proposed an example of such prolongation in, for example, a hydrodynamic bearing. In the hydrodynamic bearing disclosed in this publication, a dynamic pressure generation groove is formed on the inner peripheral surface of a sleeve supporting a circumferential surface of a rotary shaft, and the rotary shaft is rotatably provided at the center of the sleeve. An end surface of the rotary shaft is formed into a spherical plane having a curvature R equal to a radius of the rotary shaft and brought into contact with a thrust bearing attached to the lower part of the sleeve. Hardened stainless steel is used for the rotary shaft, and titanium carbide is used for the thrust bearing. Further, a fluorine group lubricant consisting of perfluoro polyester used for a lubricant is contained in the circumference of the rotary shaft as a lubricant for a bearing.
When the rotary shaft rotates, the lubricant produces a predetermined pressure by the pumping effect generated in the dynamic pressure generation groove on the inner wall of the sleeve, and the side surface of the rotary shaft and the inner wall of the sleeve rotate without being in contact with each other. Furthermore, the lower surface of the rotary shaft and the thrust bearing rotate in such a manner that the end portion of the rotary shaft is substantially in point contact with the thrust bearing. However, since titanium carbide whose hardness is high is used for the thrust bearing and the fluorine group lubricant intervenes, the abrasion resistance of the thrust bearing is increased where it is in point contact.
In this manner, the durability can be improved by a combination of reinforcement of materials of members having parts which are in point contact and the lubricant.
The above-described prior art, however, has the following drawbacks. That is, when the rotary shaft rotates, an oil film of the lubricant which is in contact with the spherical end surface is sucked toward the outside along the end surface by centrifugal force. Moreover, the oil film at the part which is in contact with the end of the rotary shaft becomes relatively thin. As a result, there occurs a problem that the abrasion phenomenon due to solid contact or frictional heat tends to be provoked.
In addition, the material cost of titanium carbide used for the thrust bearing supporting the end of the rotary shaft from the lower side is high and the workability is also inferior, which results in high cost. Additionally, although the hardness is Hv 4000 to 2500, which is higher than the hardness of general metal materials, the coefficient of friction with SUS or the like is high. Therefore, there is also a problem that abrasion at the end portion of the rotary shaft tends to occur.
In view of the above-described actual status, it is an object of the present invention to provide a hydrodynamic bearing for a motor, which includes a rotary shaft and a thrust bearing with good abrasion resistance and low price.
To achieve this aim, the present invention takes the following measure in order to suppress generation of deficiency of an oil film by designing a lubricant itself or a flow of the lubricant, for example.
According to a first aspect of the present invention, there is provided a hydrodynamic bearing for a motor, comprising:
a rotary shaft having a convex portion provided at one end portion thereof;
a sleeve which rotatably supports the rotary shaft, and having herringbone grooves formed in an inner wall thereof;
a thrust bearing provided in contact with the end portion of the rotary shaft on the convex portion side, which restricts movement in an axial direction thereof; and
a lubricant sealed in a space surrounded by the rotary shaft, the sleeve and the thrust bearing.
As a result of this aspect, when the rotary shaft rotates, the lubricant in the vicinity of the spherical plane of the convex portion rotates in a gap between the rotary shaft and the thrust bearing by its viscosity and the affinity to the rotary shaft, and the negative pressure in the vicinity of the center of the rotary shaft becomes small by decreasing movement of the lubricant sucked toward the outside by centrifugal force. Further, movement of the lubricant stops by achieving a balance between the centrifugal force and the negative pressure, thereby preventing deficiency of an oil film.
According to a second aspect of the present invention, there is provided a hydrodynamic bearing for a motor comprising:
a rotary shaft having a spherical plane provided at one end portion thereof;
a sleeve which rotatably supports the rotary shaft, and having herringbone grooves formed in an inner wall thereof;
a thrust bearing provided in contact with an end portion of the rotary shaft on the spherical plane side, which restricts movement of the rotary shaft in an axial direction thereof; and
a lubricant sealed in a space surrounded by the rotary shaft, the sleeve and the thrust bearing,
wherein a diamond-like hard carbon film is formed at at least one of a contact surface of the rotary shaft and a contact liquid level of the thrust bearing.
As a result of this aspect, the duration of life of the rotary shaft or the thrust bearing can be prolonged and the load can be reduced by utilizing a characteristic of the high surface hardness, small coefficient of friction and small affinity of the diamond-like hard carbon film.
According to a third aspect of the present invention, there is provided a hydrodynamic bearing for a motor comprising:
a rotary shaft having a spherical plane provided at one end portion thereof;
a sleeve which rotatably supports the rotary shaft, and having herringbone grooves formed in an inner wall thereof;
a thrust bearing provided in contact with an end portion of the rotary shaft on the spherical plane side, which restricts movement of the rotary shaft in an axial direction thereof; and
a lubricant sealed in a space surrounded by the rotary shaft, the sleeve and the thrust bearing, ultra-fine diamond particles each having a particle size of 1 to 10 nm being added to the lubricant.
As a result, when the ultra-fine diamond particles each having a particle size of 1 to 10 nm intervene between the rotary shaft and the thrust bearing, the rotary shaft and the thrust bearing can be prevented from being directly brought into contact with each other.
The hydrodynamic bearing for a motor according to the present invention is designed as described above.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.